hss: ?? If you talk for 7 minutes with Carrier B, you get charged for 420 seconds (70 increments of 6-seconds each) for a total of $0.07.
Obviously Carrier A will always be cheapest (since they all have the same price, while Carrier A has the smallest increments). You need to demonstrate how much the difference is between carriers and how you arrived at your conclusion.
Also, all carriers bill calls in fractions (i.e. no rounding).